score = int(input('88'))
degree = 'DCBAAF'      #[90,99]区间和100都对应A
if score >= 100 or score < 0:
    print('wrong scor .must between 0 and 100.')
else:
    index =(score - 60) // 10
    if index  >= 0:#这里对应60分以上的成绩
        print(degree[index])
    else:
        print(degree[-1])#60分以下，对应F